This invention was made in the course of, or under, a contract with the United States Energy Research and Development Administration. It relates in general to photovoltaic conversion of solar energy to electricity and, more specifically, to a silicon semiconductor material suitable for use in such photovoltaic devices.
For several years an intensive effort has been underway to develop highly efficient photovoltaic cells for conversion of solar energy to electricity. The primary interest has been silicon doped with p- and n-type dopants to provide a p-n junction. Phosphorus is a well-known n-type dopant for silicon. Each atom in a pure silicon crystal has four valence electrons which are shared with adjacent silicon atoms in covalent bonding. When silicon crystals are doped with phosphorus which occupies the same lattice sites but has five valence electrons, the doped crystals contain electrons in excess of a pure crystal, one for each of the phosphorus atoms. At a given temperature many of these excess electrons are separated from the phosphorus atoms by thermal energy and are free to wander in the crystal rendering it an electron-conducting n-type semiconductor. Analogously, when silicon is doped with boron which has only three valence electrons, there will be one electron too few to complete the covalent bonding in the vicinity of each boron atom. This electron vacancy or "hole" appears to the lattice to be positively charged because an electron would normally occupy that site. The hole then is a positive charge carrier which, when thermally detached from the boron impurity, is free to wander in the crystal making it a hole-conducting p-type semiconductor. When a p-type and n-type semiconductor are joined together crystallographically, electrons in the n-type portion diffuse across the joining boundary into the electron deficient p-type region until a voltage equal to the sum of the diffusion potentials of the holes and electrons is established across the boundary, or p-n junction. In accordance with the energy band representation of semiconductors, by virtue of the electron and hole flows, the conduction and valence bands in the p-type material rise in energy relative to those in the n-type region. Because the Fermi level, which represents the electrochemical potential, was originally higher in the n-type crystal, the level is equalized within the joined crystals and across the p-n junction. This equalization is a result of the n-type material feeding electrons to the p-type material and the p-type material feeding holes to the n-type material. The p-type portion now contains a disproportionate amount of electrons and is at a more negative potential which causes a permanent electric field across the junction. If light of greater energy than the valence to conduction band gap falls upon the crystal, it may be absorbed by valence band electrons which are thus excited to the conduction band leaving a vacant electronic state -- a hole in the valence band. Under the influence of the electric field, the photoexcited electrons will be driven toward a lower energy state, the n-type portion of the crystal, while the holes move toward a lower energy state which for them is in the p-type region. Accordingly, a photovoltage is created and a current can flow in an external circuit.
Not all solar energy is appropriately absorbed by photovoltaic cells. Due to selective absorbence and junction losses as well as conversion of much of the absorbed energy to heat, the maximum theoretically obtainable efficiency for a silicon photovoltaic cell is about 22%. While cell efficiencies as high as 16% have been obtained in silicon solar cells developed for extra-terrestrial application, such cells were fabricated from expensive single crystal silicon. The difficulties associated with mass production of single crystal silicon have prevented silicon photovoltaic cells from being cost effective for large scale terrestrial purposes. Since size, weight and efficiency for ground-based solar energy installations are not so critical as for space applications, some sacrifice in efficiency can be tolerated, and the cost of the cell can be significantly reduced. Accordingly, a moderately priced, moderately efficient solar cell has long been needed.